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Topic Review (Newest First)

04-02-2007 06:23 PM

sel2real

in the past 7 years, and 7 demon carbs purchased, 6 of the 7 have had issues (debris inside, idle feed circuits too big, etc) and 1, the ONLY road demon i bought, worked out of the box (minor tuning, of course). all in all, once all of the issues are worked out, the carb ran very good, if not, better than most holleys i've ever dealt with. there's a lot of information about people's findings with barry grant's carbs on this forum... all you need to do is search for it!

my 0.02

04-02-2007 05:08 PM

irishboom

yup

I had the same problems tuning my carb...one question I have is...what type of intake is it?...and are you running a spacer and what type if you are?...I was running a single-plane intake and an open spacer...this made the demon run like *** when my 650 dp holley ran ****...so these are key questions in the tuning process of the carb for sure...good luck...ps when I took the thing apart...I had all kinds of flashing and metal in the circuitry...so I would def. rip it down and look for that type of stuff too...once we put a "cloverleaf" spacer in and cleaned all the metal out of it...we vaccum tuned it and it ran mint ever since!!!

08-13-2006 02:59 PM

Mad Maggot

LOL @ the religious metaphor

There's quite a few reasons why you could be getting an off-idle stumble, Demons seem to be more prone to this. It's probably in need of some tuning. What engine/tranny/carb size/gear ratio do you have? I know that a heavy car with an auto tranny and highway gears can have a stumble with just about any carb, sometimes the squirters and pump cam need to be adjusted to cover this up.

Does the car idle well? Does it run smoothly at 1200-1500 RPM, or is there a constant lean condition (white smoke, choppy sound) at these RPM? Are your butterflies set to the proper positions? If it idles well, and runs well at a constant RPM, but hesitates or stumbles only when accelerating, then it's likely in need of some squirter/pump cam tuning.

I don't claim to be a carb wizard, but I've tuned my fair share. Here's a few great guides that can help with further tuning (written by experts, of course). They talk about tuning Holleys, but Demons work in pretty much the same way.

Mine is still possesed. I did check the new gaskets visually, and I can't see anything at all wrong with them. I have even seen where Tech@BG has posted that the gaskets are interchangeable. A smart forethought in the design no doubt.

Maybe I'm going to the wrong people. I need an exorcist.

08-12-2006 03:17 PM

Mad Maggot

Yes I managed to fix the problem by drilling out the two holes that should have been drilled from the factory. (Shown in the pictures above) It idles great and light acceleration is responsive but I still seem to have a hesitation when I punch it off the line. I haven't played with the squirters yet so I suspect that this should fix the problem, though I will probably need to go with the big 50cc pump. As for the problem you described, I noticed that when I put the metering block gaskets from a Holley and Demon together they don't always have the same hole alignment. I'm not sure if I mismatched models when I did this, but it wouldn't hurt to check if all the holes in your gaskets line up with the holes in your carb. Also don't forget to check if there are holes that are completely covered by the gasket. It's possible that something is blocked off and you are not getting the right amount of fuel.

Another thing to try is adjusting the butterfly positions on both the primary and secondary side. My experience with Demons has taught me they can be very finicky if they are not set just right.

08-11-2006 04:38 PM

hotrodf1

Hello hello out there.

Just curious if this problem was ever fixed for you! I am having the same issues with a demon I just acquired from my neighbor. He was having an off idle stumble really bad like you've described.

He never did get it to work again on his car. Originally, the carb he worked well for him. THen I think he put new gaskets in and it has never been the same since. I did notice that the new gaskets are blue. Does this mean they are HOlley gaskets??? Will they work??

Any more help from BG TECH would be much appreciated.

THanks in advance for any help you can give!

jer

07-25-2006 10:30 PM

Mad Maggot

Well, curiosity got the best of me, and I decided to take the carb apart and do a more thorough check of all the parts - against my better judgement seeing as how this carb is still under warranty. It's a damn good thing I did, Once I took off the metering blocks, I found metal shavings embedded in the metering block gasket where it mates to the main body, and some of those shavings came loose once the gasket was removed. The same thing happened to the secondary side as well. Come on, Barry Grant. I know you can do better than this - better give the quality control guys a smack upside the head. What if one of those shavings found it's way into a main bearing? ... ... It looks to me like they came from where the entrances to the main passages to the boosters in the main body were reamed out to make the funnel shape, as there was still some small shavings still not quite detached from there.

I had some time today so I got a hold of an air compressor and an air nozzle with a fine rubber tip, and went to work blasting all the passages with 120 PSI of compressed air, to make sure none of the metal shavings made it into any of the orifices. Once I was sure the metering blocks and main body were free from debris, I decided to have a look at the idle circuits to see if I could find anything that might be causing me trouble. At first I did not see anything wrong, so I decided to blow air through the jets to see where the fuel was able to go. It turns out after some deductive reasoning and some time blowing air and blocking various orifices with my finger that I could not get any air to come out of the orifice leading to the transfer slot or idle discharge port (the part that is regulated with the idle screw). I tried blowing air backward through this orifice and it would not go anywhere. This led me to believe there was no way for the fuel to get to the transfer slot or idle discharge port. I didn't seem to have this problem with the secondary side. After some looking, I discovered a potential problem with the primary metering block, as shown in the pictures I have attached.

The secondary metering block has a hole drilled in the orifice shown in the red circle, while the primary metering block does not. My guess is that the hole is supposed to be where the fuel goes from the main well into the idle circuit, and if that is the case, the entire fuel supply of the primary idle circuit is cut off. This would certainly explain my dead lean problems. I would appreciate it if you would confirm if this is in fact what is causing the problem. If so, I'm going to need a replacement metering block under warranty, unless there is some way I can drill the holes myself (though I don't have much confidence in my ability to be that intricate with a drill). I would also need a precision drill bit set to have the proper size hole, and that could be costly. Please inform me on how I should proceed.

07-24-2006 10:55 AM

Tech @ BG

Mad Maggot,

Sorry about that, I've been out of town. You've got a PM.

07-23-2006 02:28 PM

Mad Maggot

BUMP!

I'm still hoping there's a way to fix this bugger, I'm not quite ready to give up on it yet

07-18-2006 09:51 PM

Mad Maggot

Ok I checked for vacuum leaks with the carb cleaner, and there are no problems to be found. So now that that is rectified, I went ahead and performed those diagnostics you mentioned. First I set the idle so it was stable and not too rich, then pulled on the throttle until it went lean and wanted to die. Pumping the accelerator pump while not moving the throttle at this point definitely made a huge difference, it increased the RPM substantially and eliminated the sputtering, then after a couple seconds it returned to the dead lean state again. I also tried this starting from a higher RPM point and dropping the throttle to the lean condition, and it basically did the same thing. The wierd part is that plugging the air bleeds (the bigger ones on the outside) didn't really do too much to help the problem, although the best I could do plugging it was two small hex keys, so I doubt if I sealed the bleeds off entirely. The choke horn was in the way and I couldn't get my fingers to cover the holes too well. If I need to totally seal the bleeds to have an effect, I will come up with a better way to do this next time.

Also, I fixed the old 650 Holley that this Demon is replacing, and I put it back on for a bit and it worked great - so no vacuum leaks for sure. If for some reason I really need to get deep into the Demon to find the problem at least I have a backup so I can drive my car I bought a rebuild kit for the Holley and I still have two bowl gaskets and two metering block gaskets left over so if I need to tear into the Demon repeatedly I at least have backup gaskets.

Thanks for the help so far, and if there's any other suggestions please let me know.

07-18-2006 05:58 AM

Tech @ BG

Mad Maggot,

I'm not wanting you to change the bleeds just yet, just to see if we can make a difference to the problem you're having. From there we can work with you to help dial the carburetor in.

07-17-2006 07:35 PM

Mad Maggot

Tech @ BG, thanks for the tips - I have not tried either of those diagnostic methods before and once I get some time probably tomorrow night (I'm very busy during the week) I'll let you know how it goes. The one thing that concerns me is that you mention playing around with the air bleeds, and since I have a Speed Demon instead of a Mighty Demon, the air bleeds can't be changed out, so I hope this doesn't turn out to be the problem. I already bought some carb cleaner and I will make sure I check for vacuum leaks prior to performing more carb diagnostics.

07-17-2006 06:57 AM

Tech @ BG

Mad Maggot,

It sounds like all of the circuits are working, but not calibrated properly for what you're combination is needing since you're able to make changes that are at least changing the way the engine is responding.

Now that youíve mentioned that you canít keep the engine at a steady RPM it should be easier to diagnose. Iíd bring the engine to whatever RPM you can keep it running at steady (say 2000) then lower the RPM. Now when the engine wants to die I want you to press on the accelerator pump arm without moving the throttle linkage. This is going to add fuel without changing the amount of air the engine sees and will confirm that you are too lean since it will help the engine to continue to run. If that is the case then take your fingers and cover the outside (idle) air bleeds in the front of the carburetor. This should add a bunch of fuel through the idle circuit and help get the engine to continue to run at this point. Where not looking to immediately fix the problem just yet, but to determine exactly whatís causing it. You can even use a couple of tooth picks to plug up those bleeds for the moment to keep the engine running. Once we see what this does we can determine what direction we need to go to get this problem rectified.

As far as your vacuum question a simple way is to spray around the carburetor, and intake with some carburetor cleaner while you have the engine running. If there is a vacuum leak the engine tone, and RPM will change.

07-16-2006 08:09 PM

Mad Maggot

Oh, one more thing I should bring up, what is a good way to check for vacuum leaks on an engine? I know my engine was working great before I changed carbs, and I haven't touched anything BUT the carb, but over the last three and a half years my car has been a great example of Murphy's Law. I really can't rule out the possibility that a vacuum leak somehow formed by way of divine intervention or something.

07-16-2006 02:46 PM

Mad Maggot

I did some more experimentation to follow the suggestions I have received here, and unfortunately the problem has not gone away.

I adjusted the screw on the primary accelerator pump arm so if the throttle blades move even just barely some fuel will come out of the squirters. It wasn't bad before but now I removed ALL the slack from the pump arm against the linkage. While I might notice a hair more responsiveness with a light throttle tip now, it still does not fix the dead lean condition once the pump shot has been used up - let me clarify this: It does not matter how long the pump shot is, if I tip the throttle gently and hold it there, so as to keep the engine revving at about 1200 RPM, it is dead lean and stays that way until I either gun it or the engine dies.

I played around with the timing, remembering my old setting, and just experimented with retarding and advancing the initial timing as well as the vacuum advance on the distributor. It seemed that my original settings are still about optimum for this new carb, because any change in the timing seems to make it run worse, especially if I advance it more.

As far as the mixture screws are concerned, I can have all four screws all the way out to the point where they are about to fall out of their holes, and the off idle lean problem still exists, although not quite as bad. It is able to run at 1200 RPM with the screws all the way out, but it is very choppy sounding and there is still white smoke from the exhaust.

I played with the butterflies a lot, starting with having the secondaries all the way closed and adjusting the idle from the primaries only, to having the primaries closed all the way and adjusting the idle with the secondaries, as well as pretty much every imaginable combination in between. My conclusion is that it doesn't seem to matter where the butterflies are, it still is not getting enough fuel off idle.

Also, I raised the primary float so high you can't see the top of the fuel level in the sight window, and it STILL is not getting enough fuel off idle. The idle itself is extremely rich where the exhaust burns my eyes and nose, but once I slowly increase the revs it goes dead lean, and this confuses me greatly.

I've never seen a carb to this day that has one single circuit that can't get enough fuel, while the rest of the circuits are fine I'm going try changing the primary jets from the stock 67's to some 74's I have in another carb here, just to see what the heck will happen. I know it will be rich as hell, but if the transition circuit is still lean after I do this then I will have to look again for a blockage or something that I may have missed before.

Tech @ BG, if you read this post, what would qualify a carb to be considered defective where it must be replaced under warranty? Does it have to be something obviously visually wrong with it such as a main body crack, or can a problem like I'm having be considered a possible defect? I've never had a problem with a Demon carb until this bugger came along. Heck, I briefly ran an 850 Mighty Demon double pumper on this same motor and it worked great, even being badly oversized, yet this 750 with vacuum secondaries is having these kinds of problems.

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